Vitreous bonded abrasive grinding wheels have been produced in the art for a long time by methods that essentially employ the steps of mixing together abrasive grains, vitreous or ceramic bond precursor ingredients (e.g. frit or oxides and silicates) and a temporary binder, placing the mixture in a mold and pressing the mixture in the mold to approximately the desired size and shape of the wheel, extracting volatiles from the pressed wheel, usually by heating the pressed wheel at a relatively low temperature (e.g. 200.degree. to 300.degree. C.), removing the wheel from the mold and then firing the wheel at a relatively high temperature (e.g. 500.degree. to 1200.degree. C.) in a furnace to form the vitreous bond and bind together the abrasive grains. The removing of volatiles from the pressed wheel before the firing step is generally done, in prior art methods, because such volatiles, introduced along with ingredients such as temporary binders, can cause bloating (non uniform expansion), rupture and distortion of the fired wheel if allowed to remain in the compressed wheel when the wheel is subjected to the high temperature firing step. The volatiles maybe water and/or organic materials. Heating the pressed wheel at a relatively low temperature has the further object of causing the temporary binder to bind together the various components of the wheel in a temporary and fragile manner so as to allow removal of the pressed wheel from the mold. This temporarily bound pressed wheel is often referred to as a green wheel. During the firing step, which generally takes place at temperatures far above the decomposition temperature of the temporary binder, the temporary binder is removed from the wheel and any residual volatile materials are expelled.
The firing of the pressed, temporarily bound (i.e. green) wheel usually is done at temperature in the range 500.degree. to 1200.degree. C. During this high temperature heating various physical and/or chemical transformations occur resulting in the formation of a vitreous or ceramic matrix that binds together the abrasive grains. It is during the firing step that pores are formed in the wheel and volume changes occur. The change in volume is often manifested in shrinkage of the wheel. Particulate materials for forming the vitreous bond matrix change chemically by reaction and/or physically by melting and/or fusing together. These chemical and/or physical changes produce a reduction in the volume occupied by the particulate material for forming the vitreous bond. Additional particulate material, other than the abrasive grain may be incorporated into the vitreous bond matrix and may act to cause a further reduction in volume. The extent of the shrinkage is in large measure dependent upon the magnitude of these changes and therefore on the amount, as well as the chemical and/or physical characteristics of, the vitreous bond forming matrix materials and other particulate materials used in making the wheel and upon the degree of porosity achieved in the wheel. Shrinkages of from 0.5% to 10% by volume are known, particularly in relatively porous wheels (e.g. 20% porosity by volume or greater). To exemplify and explain this matter of shrinkage one can visualize the particulate material for forming the vitreous bond matrix of the wheel as being glass beads. Placing these beads in a container to fill it even with the most efficient packing of the beads, leaves spaces unoccupied by the beads. The melting of the beads to form liquid glass results in a volume of glass less than the volume occupied by the beads. This change (i.e. reduction) in volume then is the shrinkage resulting from the melting of the glass beads.
Undersized wheels, out of tolerance central mounting holes for the relatively porous wheels, separation of mating segments (e.g. cores from rims) and even cracking or distortion of vitreous bonded grinding wheels have been some of the observed consequences of wheel shrinkage during firing. Some of these problems (e.g. undersized wheels) have been overcome in the art, by making the green wheel of a size sufficiently larger than the fired wheel to compensate for shrinkage or by making the fired wheel larger than the desired finished size and then machining the wheel to the proper size. Because shrinkage has been found in the art to be difficult to control in relatively porous wheels (i.e. to obtain consistent, reproducible results) the making of the green wheel of a size sufficient to compensate for shrinkage has not been found to be an all together reliable answer. A more acceptable answer to shrinkage has been the preparation of the vitreous bonded grinding wheel to a size larger than required and then machining the wheel to the correct size. However even here problems remain. The correction of out of tolerance mounting holes, even by machining, has been found to be a difficult problem. Machining vitreous bonded grinding wheels to size adds steps and cost to their manufacture. Some vitreous bonded grinding wheels, especially those produced with expensive abrasive grains such as diamond and cubic boron nitride, are made with a vitreous bonded abrasive rim encircling a vitreous bonded core containing inexpensive abrasive grain or no abrasive grain. In the known methods of making these wheels, shrinkage has been observed to cause separation of the core from the rim and even distortion of the wheel. Such problems result in scrap wheels (i.e. wheels unsuitable for use) and increased cost for these already expensive wheels.